1. Field of the Invention
The present invention relates to a magnetic black toner for electrophotography for visualizing an electrostatic latent image formed on the surface of a photoconductive insulator such as a photoconductor drum in an electrophotography method or the like, to a magnetic two-component developer for electrophotography containing the same, to an image forming apparatus, and to an image forming method.
2. Description of the Related Art
Electrophotography is one conventional method for visualizing electric image data on recording paper or the like. In the electrophotography method, an electrostatic latent image is formed on the surface of a photoconductive insulator (photoconductor drum or the like; the latent image is developed and visualized by electrically attaching a one-component toner, which acquires charging with a developing machine equipped with a contact charging mechanism such as a blade, or a two-component toner, which acquires charging by being contacted with a carrier; and the visualized toner image is then transferred on the recording paper or the like. Finally, the transferred toner image is melted and solidified (fused) to produce a printed image.
The formation of the toner image on the surface of the photoconductive insulator may be performed as follows, for example. First, a uniform electrostatic charge is imparted to the surface of the photoconductive insulator (such as the photoconductor drum) by corona discharge or the like, and the electrostatic latent image is formed by radiating an optical image on the photoconductive insulator by suitable means. Next, the toner image is formed by attaching a charged toner to the electrostatic latent image using the electric attractive force of the electrostatic latent image. As the toner for developing the electrostatic latent image, particles which are produced as follows may be used. That is, a colorant and an additive such as a magnetic material and a charge control agent or the like, are dispersed in a binder resin made of a natural or a synthetic polymer material, and the binder resin dispersing the colorant therein is ground to produce fine particles having diameters from about 1 μm to about 30 μm.
Methods for fusing the toner image to the recording paper or the like, include those that involve melting the toner using the pressure, those in which the toner is heated, and those that use a combination of pressing and the heating, in all of which the molten toner becomes fused upon solidifying, as well as a method of irradiating the toner with let, and then solidifying and fusing the melted toner. The toner adhering to the recording paper forms a semipermanent image, and such printed images have become an indispensable part of modern society. Here, the selection of the colorant used for the toner in visualization is important, as it greatly affects image quality.
Images of electrophotography vary from “black and white” and “single color” to “full color” images, with full color images becoming increasingly common. However, the market for “black and white” images remains large, and even in full color equipment it is common that image formation is performed using four colors, that is, black in addition to yellow, magenta, and cyan. Thus, black material remains necessary in common electrophotography.
The black material of the toner is used as the toner after processing as follows. First, the black material is dispersed in a resin by mixing and kneading the black material with the resin. Then, the resin dispersing the black material therein is ground and classified to create a unified resin having a desired grain diameter, while organic and inorganic particles are added as determined necessary to achieve the desired fluidity, charge carrying capability, adjusting resistance, or the like. The material thus processed is then used as the toner. Conventionally, as the black material for electromagnetic toners, a magnetic material, commonly magnetite particle powder, has been used. In particular, a toner in which the black material is 50 mass % or more are commonly used in a magnetic one-component toner or the like as used in a one-component system process. Such a high content enables a sufficient degree of blackness to be obtained. Moreover, in a non-magnetic toner, carbon black particle powder and the like have been widely used as the black material.
Meanwhile, in recent years, there have been tests of a system in which an identification mark having a magnetic signature is printed on documents such as checks, convertible securities, bills, tickets, and the like, with the object of preventing forgery or alteration by means of a magnetic one-component toner development method using a magnetic toner containing a magnetic material therein. Moreover, a printed magnetic identification mark has been used as a check in the United States and in some European nations. Generally, such a system is referred to as a magnetic ink character recognition (MICR) system, and an MICR toner printer having a function of enabling reading with the MICR system has been placed on the market. The toner for printing an MICR font using an electrophotography system is called as a magnetic toner for an MICR printers (there is a case where the toner is simply called as an MICR toner), and such toners are disclosed in, for example, Japanese Patent Laid-Open Publications No. Hei 2-134648 and No. Hei 5-80582, and in U.S. Pat. No. 5,034,298. The MICR toner printer is a small-sized magnetic printer mainly, and uses the same process as employed in a conventional magnetic one-component toner. However, in order to enable reading by an MICR reader, a magnetic material having, in addition to the conventionally desirable characteristics, a predetermined remanent magnetic force and coercivity is used.
As described above, small-sized printers using magnetic one-component toners are currently the most commonly employed MICR printers. On the other hand, as the number of checks, bills, and the like in circulation is huge, a system capable of performing printing at a high speed and in a large quantities is desired. However, it is difficult for the above-mentioned small-sized printer to perform the high speed/large-quantity processing because of its magnetic one-component process, and a two-component system developer using a carrier is a more desirable process. Moreover, preferable magnetic properties are indispensable in MICR toners. However, if a magnetic one-component toner containing a magnetic material of 50 mass % or more is used as a two-component toner, then magnetic adsorptive power is generated in addition to electrostatic adsorption with the carrier, and a desired amount of toner adhesion is not obtained on printed matter to produce a failure of reading. Accordingly, it is necessary to lessen the content of the magnetic material necessary for the MICR property in the in two-component development for MICR.
However, there is a problem in which, when the quantity of the magnetic material is reduced, printed images appear reddish brown to deteriorate the printing quality because of the reddish brown color of the magnetic material.
Accordingly carbon black is sometimes added to the toner to prevent reddish-browning. However, although carbon black is a material having a very high masking rate and a high degree of blackness, carbon black particle powder is difficult to handle and complicates manufacturing of the toner because the carbon black particle powder are ultrafine particles of a bulky powder. Moreover, there are cases wherein the fusing property of the toner is reduced due to the increase of the viscosity of the toner caused by the filler effect according to the presence of the carbon black. Furthermore, there is a problem in that toner resistance is decreased due to the addition of the carbon black having a high electrical conductivity in addition to the magnetic material, and that fogging results.
Moreover, although it is also possible to use magnetite powder particles having weak magnetism in order to prevent reddish-browning, additional problems result when the magnetite powder particles are employed. For example, because the magnetite particles have strong cohesive forces among particles, the magnetite particles have inferior dispersibility, to the extent that the manufacturing property and the stability of the resistance and the charging property at the time of toner production can be impaired. Moreover, when the magnetite is used under a high temperature condition during the manufacturing process, the fusing process, or the like, the color of the magnetite may change from black to brownish-red.